The present invention relates to a semiconductor integrated circuit and its evaluating method.
Generally, in an evaluating test which is performed for measuring the delay time of the critical path of a semiconductor integrated circuit, mainly DC and AC tests have been used. In recent years, there has been an increase in the prices of semiconductor integrated circuits, following the attainment of high performance and high integration for the semiconductor integrated circuit and the increase of its size. Thus, AC test selection has become particularly necessary for the inspection of a single semiconductor integrated circuit.
On the other hand, in order to perform an AC test similar to that for a device in the single semiconductor integrated circuit, a great number of facilities and processes are needed. For this reason, its trade-off relationship with the above-noted AC test selection for the inspection of the single semiconductor integrated circuit is one of the problems to be solved.
To take one example (conventional example 1) hitherto performed, in a typical semiconductor integrated circuit, especially in a logical LSI, automatic arranging and wiring are performed with the net list of a desired function as an input and if there is no problem found with its delay analyzing result, the process enters a wafer step and then advances to an inspecting step.
During the AC test for a sample to be measured, normally a test pattern used for function testing is created, and the operation of a desired frequency is tested for a desired evaluating function by a LSI tester, and so on.
To take another example (conventional example 2), a ring oscillator is installed beforehand in the semiconductor integrated circuit separately from an original function circuit, and by measuring the frequency of the ring oscillator by using a frequency counter, this is used instead of an AC test for the original function circuit. Referring to FIG. 12, which is a conceptual view showing the substrate arrangement example of the semiconductor integrated circuit in this conventional example, there is arranged a ring oscillator 29 corresponding to a function circuit included in a semiconductor integrated circuit 27, and there are provided signal pulling-out pads 30 and 31 corresponding to the ring oscillator 29. The content of a specific example corresponding to this conventional example 2 is disclosed in JP-A-160377/1992.
As another conventional example (conventional example 3), there has been used a method for measuring and evaluating the delay time of a part of the function circuit instead of an AC test for the function circuit in the semiconductor integrated circuit. This method will be described by referring to FIG. 13, which is a conceptual view of this semiconductor integrated circuit. In the evaluating method shown in FIG. 13, measuring is performed for a delay time T1 in a bus 36, which passes through a circuit 34 to be measured in a function circuit 33 included in a semiconductor integrated circuit 32, and a delay time T2 in a bus 37, which does not pass through the measured circuit 34, and by referring to a measured time difference between these two (T1-T2), a delay time in the measured circuit is evaluated.
However, the foregoing semiconductor integrated circuits of the conventional examples 1 to 3 have drawbacks described below.
In the case of the conventional example 1, in its evaluating method, it is generally difficult to create a test pattern, which corresponds to an original function test in the semiconductor integrated circuit, and measuring devices like a LSI tester, and so on, used for such a purpose, are very high in prices.
In the case of the conventional example 2, the built-in ring oscillator is very small in its form compared with the original function circuit, and thus when there is large non-uniformity in the characteristics of various transistors arranged in the semiconductor chip, it is difficult to obtain a correlation with an AC test characteristic for the function circuit. Consequently, the test function which is used instead of an AC test for the function circuit is lost.
Furthermore, in the case of the conventional example 3, if the number of measured circuits in the semiconductor integrated circuit is small, as in the case of the conventional example 2, it is difficult to obtain a correlation between the measuring results of the measured circuits and the AC test characteristic of the entire function circuit. If there are a number of measured circuits, the overhead of an added test circuit is large and the size of a semiconductor chip is increased.